Dative
When ammonia (NH₃) reacts with a hydrogen ion (H⁺) to form the ammonium ion (NH₄⁺), the bond formed is a dative bond, also known as a coordinate covalent bond. In this type of bond, both electrons in the bond come from the same atom—in this case, the nitrogen atom in ammonia donates a pair of electrons to bond with the hydrogen ion.
So, the correct answer is:
B. Dative